164 MOSQUITOES OF NOETH AMERICA 



effect was apparent until the larvae were about to pupate ; then the worms, hav- 

 ing probably completed their parasitic stage in the mosquito, leave the host. 

 They break forth from the posterior end of the body by perforating the mem- 

 brane surrounding the anus. The larger worm leaves first, the smaller follow- 

 ing a little later, and the mosquito larva dies as a result of the injuries. The 

 Mermis larvae died some hours after leaving their host. In one catch all the 

 mosquito larvae were infested, but parasitized lots of larvae were rare ; only two 

 out of 26 lots were infested and these came from water in trees. No parasites 

 could be found in adult AMes calopus and Gendre considered the infestation 

 accidental. 



INSECT ENEMIES. 



Many insects and nearly all of the predatory aquatic forms that are able to 

 master mosquito larvae, will feed upon them and they form an important article 

 in the diet of many species. Most of these carnivorous insects, however, are 

 not prolific breeders, and they seldom deposit their eggs except in rather per- 

 manent pools; as a consequence their importance as mosquito destroyers is 

 not so great, though nevertheless considerable. Mosquitoes breed practically 

 unmolested in artificial receptacles for rain-water and in transient pools. 



The larvas of the predatory water-beetles of the families Dytiscidae and Hydro- 

 philidae eat many mosquito larvae, although from their great activity these often 

 escape and the beetle larvae derive the greater part of their subsistence from 

 other forms. 



Dr. W. E. Britton, of New Haven, Conn., who has made careful study of the 

 mosquito problem in Connecticut, has found that the larva of Hydrophilus 

 oMusatus Say (Schwarz det.) is one of the important natural enemies of mos- 

 quito larvae in the vicinity of New Haven. 



One of us (Howard) has recorded an observation in the course of which he 

 took a half gallon of water from a stagnant pool which was teeming with aquatic 

 insects, including hundreds of mosquito larvae. Among these insects were three 

 Hydrophilid larvae and in the course of a week these three larvae devoured practi- 

 cally all of the other animal life in the jar in which the half gallon of stagnant 

 water had been placed. 



Dr. J. B. Smith thinks that the larvae of the Dytiscidae are among the greatest 

 enemies of mosquitoes. He is inclined to attribute the comparative freedom 

 from mosquitoes of the hilly section of New Jersey very largely to these insects, 

 and he quotes an interesting observation by Doctor Johnson in support of this 

 conclusion. He also notes an observation by Mr. Viereck in which one good- 

 sized dytiscid larva was seen to devour 434 mosquito larvae in two days. Galli- 

 Valerio and Eochaz de Jongh found that an adult water-beetle, Dytiscus mar- 

 ginalis, although it had been previously fed with meat, destroyed great numbers 

 of mosquito larvae and pupae. Dr. Adolf Eysell states that a single, nearly full- 

 grown, larva of Acilius sulcatus destroyed more than forty mosquito larvae over 

 night. 



Dr. Smith states that the Gyrinidae or so-called " whirligig beetles,'^ which live 

 nearly wholly at the surface of the water, are great destroyers of mosquito larvae ; 



